Technical Field
Embodiments of this disclosure generally relate to a fixing device to fix an unfixed toner image onto a recording medium, and to an image forming apparatus incorporating the fixing device, such as a copier, a printer, a facsimile machine, or a multifunction machine having two or more of copying, printing, and facsimile capabilities.
Related Art
Image forming apparatuses, such as copiers or printers, typically incorporate a fixing device employing electromagnetic induction heating to reduce startup time of the image forming apparatuses, thereby enhancing the energy efficiency. Such a fixing device employing electromagnetic induction heating includes, e.g., a support roller (or heating roller) serving as a heat generator, an auxiliary fixing roller (or fixing roller), a fixing belt stretched over the support roller and the auxiliary fixing roller, an induction heating unit (or induction heater) facing the support roller via the fixing belt, and a pressing roller to contact the auxiliary fixing roller via the fixing belt. The induction heater includes, e.g., an excitation coil wound in a longitudinal direction of the induction heater, cores to direct an alternating magnetic flux arising from the excitation coil to the heat generator, and a holder (or coil guide) to hold the excitation coil and the cores.
The fixing belt is heated by the induction heater at a position where the fixing belt faces the induction heater. While a recording medium carrying a toner image passes through the auxiliary fixing roller and the pressing roller, the heated fixing belt heats the toner image formed on the recording medium, and accordingly, the toner image is fixed onto the recording medium.
Specifically, a high-frequency alternating current supplied to the excitation coil forms an alternating magnetic field around the excitation coil, which generates eddy currents on and around the surface of the support roller. When the eddy currents are generated around the support roller serving as a heat generator, the electrical resistance of the support roller leads to Joule heating of the support roller, thereby heating the fixing belt stretched over the support roller.
In such a fixing device employing the electromagnetic induction heating, the heat generator is directly heated by electromagnetic induction. Accordingly, compared to a typical fixing device using a halogen heater, the fixing device employing the electromagnetic induction heating has a higher heat-exchange efficiency and therefore the surface temperature of the fixing belt can be increased to a desired fixing temperature more efficiently, that is, with less energy and a shorter startup time.
To further enhance heat generation efficiency, it is effective to form a magnetic path that perfectly directs the magnetic flux arising from the excitation coil to the heat generator. Hence, for example, a side core that forms the magnetic path is insert-molded in the holder that holds the excitation coil so that ferromagnetic cores including the side core are exposed at the holder. With this configuration, the ferromagnetic cores can be positioned closer to the fixing member, thereby enhancing heating efficiency. However, when the side core is insert-molded in the holder, the side core may be broken if it is warped. Such a broken side core cannot evenly direct the magnetic flux to the heat generator, thus hampering uniform heating efficiency.
One approach to such side core breakage involves providing a side core having a center thicker than both ends, as with side core 64a illustrated in FIG. 20. Such a configuration reduces warping, thereby preventing the side core 64a from being broken when the side core 64a is insert-molded in a holder. However, in this case, the volume of the side core 64a is reduced by notches 64b as illustrated in FIG. 21, which is a side view of the side core 64a along a direction indicated by arrow Z in FIG. 20. The result is that heat generation efficiency is also decreased, with less magnetic flux directed by the side core 64a. 